The present invention relates in general to closed fluid flow systems and is particularly directed to the accurate measurement of a variable pressure in a catheter-tubing-transducer system by damping out system resonance.
The catheter-tubing-transducer system typically used in various health-related applications is an underdamped, second-order, dynamic fluid system similar to a bouncing tennis ball. The catheter, which is inserted into a patient, and tubing are filled with a liquid, typically a saline solution, to permit hemodynamic pressures to be transmitted as pressure pulses through the liquid-filled catheter. The pressure waves propagated in the system due to a patient's heartbeat have a characteristic frequency, with the propagation of these waves related to the system's inherent damping coefficient. In terms of mechanical parameters, this type of second-order system can be described in terms of three parameters: elasticity, mass and friction. Elasticity refers to the stiffness of the system, normally caused by the flexibility of the transducer diaphragm. This elasticity can be changed by air bubbles, compliant tubing, or other elastic elements in the system. The mass of the system is the fluid mass moving in the catheter and interconnecting tubing. Frictional forces arise at the inner surface of the catheter and tubing as the fluid within is displaced with each pulsating change in blood pressure. In this type of underdamped, second-order system, the aforementioned three system parameters determine two measurable parameters, i.e., the system's natural frequency and damping coefficient. The natural frequency refers to how rapidly the system oscillates, while the damping coefficient refers to how quickly the system comes to rest.
Frequently it is necessary, particularly in the case of one who is critically ill, to continually and accurately monitor a patient's heartbeat and blood pressure characteristics. Electromanometry systems for monitoring and recording hemodynamic pressures are gaining increasing importance in modern medicine. These systems transform hemodynamic pressures into observable and recordable electronic waveforms representing the periodic pulsations of blood transmitted as pressure pulses through the liquid-filled catheter to a transducer. From the parameters thus measured and recorded, important diagnostic data concerning a patient's condition is made available.
In this type of system, yet another system parameter, the frequency bandwidth of the system, must be considered. The system's frequency bandwidth provides an indication of the accuracy of the pressure readings, with a high bandwidth representing a fast system response capable of providing accurate and reliable hemodynamic pressure measurements. Those systems with the best dynamic response are those having a high natural frequency which allows for great latitude in system damping coefficient. However, compliance in the system, such as introduced by a small air bubble, increases the damping coefficient and decreases the natural frequency. In addition, these systems possess a characteristic resonance which results in a tendency to amplify pulsations in the region of the system's natural resonant frequency much more than pulsations having other frequencies. This results in a form of distortion in measuring the pulsating waveforms known as "harmonic ringing". The resonance of this type of closed fluid flow system seriously degrades the accuracy of pressure variation measurements in such systems.
The prior art discloses various attempts to compensate, or correct, for the characteristic resonance of an electromanometry system. These approaches have included electrical compensation systems which are generally expensive and complicated. Other approaches have employed hydraulic damping devices with varying degrees of success.
One such approach which makes use of a variable series hydraulic resistance in the system is shown in FIG. 1. This device 10 is coupled in the system between the transducer and a flush device by means of first and second couplers 16, 18, where fluid flow is from left to right in FIG. 1. The device includes main tubing 12 to which is securely coupled, so as to be integral with, a housing 14 with an inlet port and an interior throughbore. Inserted by means of threads within interior throughbore is a threaded shaft 20 having at one end a knob 22 and an opposite leading tapered end 26 which terminates in tip 28. By rotating knob 22, shaft 20 may be longitudinally displaced along the interior of cylindrical housing 14 to permit tip 28 to be selectively positioned relative to the fluid flow. By selectively varying the series resistance in the system, the damping therein may be controlled for reducing resonances therein. This approach, however, results in reduced high frequency response and limited system measurement accuracy.
Another approach is described in U.S. Pat. No. 4,335,279 to Reynolds et al involving a variable parallel capacitance damping approach as shown in FIG. 2 herein. This device 40 includes a plastic T-coupling member 42, to the opposite ends of which are respectively coupled conventional female and male luer fittings 44, 46. Female luer fitting 44 is in the direction of a transducer (not shown), while male luer fitting 46 is in the direction of the patient. The T-coupling member 42 includes a cylindrical housing 54 having a threaded interior throughbore and is coupled to the main tubing by means of an inlet port 48. A side port 50 with an end cap 60 is coupled to the housing 54, in which the threaded shaft 56 is positioned by means of engaging threads. Shaft 56 includes a knob 58 for the rotation thereof. A compliant air cavity is provided on the interior of side port 50 and is connected through a variable impedance device, i.e., shaft 56 with leading tapered end 52, which is coupled in parallel to the liquid-filled catheter of the electromanometry system. By varying the hydraulic impedance through which the compliant air cavity is coupled to the system, the characteristic impedance of the system may be matched. The purpose of the invention is to provide a wide range of control for purposes of hydraulically matching the characteristic impedance of an electromanometry system. This approach, however, provides the parallel capacitance with only limited system isolation and requires a rather complicated, continuously adjustable device which increases the cost of these systems. U.S. Pat. No. 3,665,948 to Hohberger, while not intended to reduce or eliminate hydraulic resonance in a fluid flow system, discloses an apparatus for limiting the magnitude of transient hydraulic pressure waves in such a system.
The present invention overcomes the aforementioned limitations of the prior art by providing a fluid damping device for a compliant system which substantially eliminates resonances in the system permitting accurate pressure measurements to be made. In addition, the present invention is inexpensive, easily implemented, and compatible with conventional catheter-tubing-transducer configurations typically encountered in electromanometry systems.